An electroluminescence (hereinafter, sometimes simply referred to as EL) element is a planar light source, and there has been known a dispersion EL element containing fluorescent powder held between electrodes. The dispersion EL device generally has such a structure that fluorescent powder dispersed in a binder having a high dielectric constant is held between two electrodes, at least one of which is transparent, and the element emits light upon applying an alternating electric field to the electrodes.
The dispersion EL device formed by using fluorescent powder has many advantages, such as a thinner shape of several millimeters or less, a planar light source, and a less amount of heat generated, and owing to the advantages, the dispersion EL device is used in various applications, such as road signs, interior and exterior illuminations, light sources for flat panel displays, such as liquid crystal displays, and illuminating light sources for large area advertising boards.
The dispersion EL element can be formed into a flexible element using a plastic substrate since no high temperature process is used. The dispersion EL element can be produced at low cost through a relatively simple process without a vacuum equipment used, and the luminescent color thereof can be easily control led by mixing plural kinds of fluorescent powder having different luminescent colors. Owing to these advantages, the dispersion EL element is applied to various display elements, such as a backlight of a liquid crystal display (LCD) device.
FIG. 2 is a graph showing luminescent waveforms of commercially available dispersion EL elements emitting white light. As shown in FIG. 2, the white dispersion EL device currently available has such a luminescent waveform that the luminescent intensity thereof is concentrated mainly to two wavelength regions.
The white dispersion EL element has various advantages as a flexible planar light source as having been described, but involves a problem where the color rendition thereof is inferior to the other white light sources, such as a fluorescent lamp. For example, in the case where a transparent medium, such as a transparent positive image, is observed on the dispersion EL element, the color reproducibility is inferior to the case where it is observed under the conventional planar light source, such as a fluorescent lamp.